Mercury Ions Mediated Phosphorus Containing Carbon Dots as Fluorescent Probe for Biothiols Screening

In this study, we report the mercury ions (Hg${}^{2+}$) mediated phosphorus-containing carbon dots (PCDs) as a selective “off–on” fluorescence probe for glutathione (GSH), cysteine (Cys) and homocysteine (Hcys). PCDs obtained by hydrothermal reaction are sensitive to Hg${}^{2+}$ ions and its fluorescence can be significantly quenched owing to the electron transfer from the lowest unoccupied molecular orbital (LUMO) of PCDs to Hg${}^{2+}$. Interestingly, the weak fluorescence of Hg${}^{2+}$-mediated PCDs could be gradually recovered with the addition of GSH, Cys and Hcys. This can be attributed to the formation of Hg${}^{2+}$–S complex due to the super affinity of Hg${}^{2+}$–sulfydryl bond. The formation of Hg${}^{2+}$–S complex extremely reduces the oxidation ability of Hg${}^{2+}$ that inhibits the electron transfer from LUMO of PCDs to Hg${}^{2+}$ and re-opens the native electron transition from LUMO to the highest occupied molecular orbital (HOMO) of PCDs. Thus, the green fluorescence of PCDs is switched on. Furthermore, the present Hg${}^{2+}$-mediated PCDs assay exhibits a high selectivity for GSH, Cys and Hcy and has been successfully used to detect the total biothiols content in human urine samples.